Note: Descriptions are shown in the official language in which they were submitted.
TITLE OF INVENTION: S~LF-ORIENTING DIRECTIONALLY
SENSITIVE GEOPHONE.
INVENTOR: Mr. Olive I B. listen, a citizen of the United
Kingdom, who is a resident of the United States, whose
post office address is PRO. Box 45101, University Station,
Seattle, We 98145 USA
TECHNICAL FIELD
This invention relates to electroacoustic detecting
devices and more particularly to particle velocity
detectors or transducer commonly known as geophones.
Utilizing ferromagnetic fluid as a suspension means for a
magnet, the resent invention may be used for sensing
seismic signals in survey systems which profile the
location and probable nature ox underlying geological
substrata.
BACKGllOUND ART
Seismic surveying or prospecting is one common
method used by geologists and others engaged in mineral
exploration activities to help determine the most probable
location of mineral deposits which are ox sufficient size
to justify the economic costs of extraction or mining.
Experience has taught that certain geological substrata
formations are more likely than others to be associated
with the presence of exploitable mineral deposits It is
by providing a survey or profile of geological substrata
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formations, both on land and in water covered areas, that
seismic prospecting demonstrates its utility.
To create a seismic profile, a seismic shock is
initiated at a selected point. This shock is reflected
from the various underlying substrata soften called
reflectors and these reflections are detected at a
plurality of locations by sensitive transducers which
translate the received signal into electrical impulses
which can be recorded on a chart. By utilizing the known
distance and direction of the various transducers from the
source of the seismic shock and by correlating the elapsed
time between the initiation of the shock and the detection
of the various reflections by each transducer, a profile
of the underlying strata can be constructed.
Vie to the high sensitivity of the transducers,
seismic prospecting is often hampered by background
interference. On land, this interference is sometimes
I called "wind noise" or "ground unrest". Problems with
interference take on added dimensions when seismic work is
done in water covered areas. Water is an efficient
propagator of sound in all directions. Also, the
interface between water and the air reflects pressure
waves and causes them to undergo a 180 degree phase shift.
In modern marine seismic systems the usual practice
comprises a seismic shock generator; a relatively small
diameter neutrally buoyant oil-filled flexible tube or
streamer which contains a plurality of spaced apart
pressure transducers, and a vessel or boat which tows the
generator and streamer through the water and which has on
board the equipment which processes the signals received
from the transducers and neuritis the seismic profile or
US chart. Despite an inherent sensitivity to seismic
signals in the water regardless of the direction of signal
travel (e.g., the wake of the towing vessel and the
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disturbance created by its propeller as well as
phase-shifted and reflected signals from the air-water
interface) the pressure sensitive type transducer, or
hydrophore, continues in widespread use primarily because
its small size allows it to fit in a tube of acceptable
diameter and its relative simplicity of design results in
low cost to the user. This is important because a
typical streamer can be in excess of 5,000 feet in length
and can contain thousands of transducer elements divided
between 96 or more receiving channels, often with up to MU
elements connected to each channel.
Various means and methods have been proposed as
improvements to the art of marine seismology. Early on,
it was discovered that interference from surface
reflections caused by the mismatch of acoustical impedance
at the air-water interface could be substantially
controlled by towing the streamer at a depth which
corresponded to one-quarter wavelength of the seismic
signal. Use of this towing depth prevents a 180 degree
phase shifted surface reflection from arriving at tune
transducer at the same time as the desired signal, but in
the opposite direction, which would thereby cause nearly
total cancellation of the desired signal. The surface
reflected signal does modify the appearance of the seismic
profile. Ilowever, as long as the frequencies used are
close to the one frequency for which the streamer depth is
exactly a quarter wavelength, the broadening of the traces
is small.
Difficulties arise when very high resolution of
reflectors is desired, corresponding to a very wide
bandwidth of received frequencies. Under these
circumstances, it is not possible for the streamer to be
at the correct depth to receive all these frequencies.
For instance, if it is to be quiet and receive the low
frequencies it must be at a considerable depth. At the
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higher frequencies needed for good resolution, the depth
passes through multiples of a half wavelength, where there
is almost perfect cancellation and consequent loss of
data. Towing a hydrophore streamer near the surface in an
attempt to receive a broad band of the higher frequencies
exposes the streamer to the action of surface waves and
causes high tow noise to be venerated.
In attempting to deal with sensitivity to
interference signals which travel horizontally, it has
been suggested that a dual spread of vertically spaced
apart hydrophore equipped streamers be utilized. The
vertical separation distance between the two streamers is
set such that a vertically arriving seismic wave will
lo produce outputs from each streamer which are one-half
period out of phase with each other. If the signals
output prom one streamer are then subtracted prom
corresponding signals output from the other streamer, any
horizontally traveling energy signals are canceled. This
is because such horizontal signals reach both streamers at
the same time and are in phase with each other. the
difficulties of keeping a single streamer several
kilometers long at a uniform depth apply with equal force
to this method where two streamers are to be maintained at
a fixed vertical distance from each other Deployment end
retrieval of such a system in anything but ~lat-calm
waters would also be difficult.
The two foregoing methods utilize pressure sensitive
3Q transducers known as hydrophores. As earlier noted,
hydrophores can be regarded as being equally sensitive in
all directions. There is a second type Of seismic
transducer which depends upon displacement or inertia and
therefore is responsive to particle velocity rather than
pressure. Such particle velocity transducers are commonly
known as geophones. A geophone typically utilizes a
magnet in conjunction with a coil which is suspended in
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the air gap of the magnet in such a way that the turns of
the coil will move at the particle velocity of the
surrounding medium and cut the lines of magnetic flux,
thereby generating an induced electromotive force.
s
Geophones have been developed which display an
enhanced sensitivity along a single axis, typically, the
vertical axis. This selectivity has been achieved by use
of mechanical bearing gimbal mountings together with slip
rings and wiper contacts as electrical connections. It
has also been achieved by suspending a circular coil with
three equally spaced springs so that, under the influence
of gravity, the coil achieves an asymmetric relationship
to a circular magnetic field. If an assembly containing
this latter arrangement is rotated about the axis of the
circular magnetic field, the three equally spaced
suspension springs allow gravity to cause the golf to
maintain its previously existing asymmetrical location
with respect to the field. The asymmetric relationship
between the coil and field provides vertical sensitivity
because a vertical movement of the coil will cut many flux
lines per second whereas a horizontal movement of the coil
will cause few, if any, lines of flux to be cut per
second.
Though both of the above geophones display a marked
lack of sensitivity to sideways approaching signals, a
feature of much value in marine seismic work, they are of
complicated design with numerous machined parts and
delicate assemblies which require close tolerances in
manufacture and, therefore, are not inexpensive enough or
rugged enough or, in some cases, small enough for viable
use in a marine environment. Also, due to an equal
sensitivity to movement in both directions along the
vertical axis, these geophones remain subject to the
problems associated with signals reflected from the
air-water interface.
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All of the foregoing discussion contemplates
streamers containing only one type of transducer, i.e.,
either all hydrophores or all vertically oriented
geophones. At least one proposal has been made (and this
in conjunction with the otherwise unsatisfactory gimbal
mounted geophone discussed above) to utilize
geophone-hydrophone pairs spaced along the entire length
of a single streamer. The purpose of such an arrangement
is to utilize the corroded principle to produce a
unidirectional response. The operation of the corroded
principle is rounded on the fact that a sound wave is a
longitudinal wave and can be described either in terms of
variations in pressure about an equilibrium point, or, in
terms of the actual displacement about an equilibrium
point of the particles conveying the wave. The difference
between the two descriptions of the waves that is here
important is that the displacement wave is 90 degrees out
I, of phase with the pressure wave. Lyons, by summing
signals at equal sensitivity from a particle velocity
transducer and a pressure transducer, a unidirectional
response can be obtained. sound wave approaching the
transducer pair from one direction causes both transducers
to produce outputs with the same polarity and thus the two
outputs add. A sound wave from the 180 degree opposite
direction causes the transducers to produce outputs of
opposite polarity which, therefore, cancel each other.
This unidirectional or corroded response would be
very useful for deep seismic profiling in the ocean
because the surface reflections could be eliminated,
making it possible to tow the streamer at any convenient
depth. The present requirement for precise depth control
to tune the receiver to the expected frequency of the
reflections would disappear, a a corroded receiver is
inherently broadband, a characteristic which also allows
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- the corroded receiver to easily handle the very wide
bandwidth of received frequencies associated with high
resolution work. Such high resolution work in the open
ocean would not be subject to the unsatisfactory
compromise between shallow towing for high resolution, and
deep towing for quietness in the presence of ocean waves
and vessel noise.
To achieve the corroded response, the geophone of
the geophone-hydro~hone pair must be self orienting The
long oil-filled streamer is quite flexible and can twist
or rotate along its longitudinal axis as it moves through
the water. It is therefore necessary that each geophone
have the means to seek and maintain an orientation to true
vertical independently of the orientation of the streamer
in which it is mounted. The means applied to achieve
self-orientation should be resistant to shock, introduce
no electrical noise during towing movements, and result i,.
an element of small diameter, simple design, moderate cost
of manufacture, and no maintenance or adjustment after it
is installed in the streamer.
DISCLOSURE OF TOE NOAH
The geophones of this invention are adapted to
provide vertical directional sensitivity together with an
ability to individually seek and then retain a vertical
orientation independent of the orientation of the tube or
streamer within which they are being towed. Further, due
to its novel design, one embodiment of this invention may,
at the time of manufacture, be configured to provide
sensitivity to particle movement along any desired axis
ranging from true vertical to true horizontal while, at
the same time remainillg relatively insensitive to any
movement not aligned with the axis of sensitivity.
Each of the directionally sensitive jounce of
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this invention are characterized by a structure winch
utilizes gravitational force to cause a magnetic field and
an associated coil to become physically and/or
magnetically asymmetrical in relation to a given direction
of sensitivity and yet remain substantially symmetrical
for all other directions ox movement. This is
accomyli~iled in the resent invention by using
ferromagnetic fluid to suspend a magnet within a chamber.
As will be seen, use of a ferromagnetic fluid suspension
provides both the properties of a spring suspension and
those of friction less gimbals.
The magnet remains suspended in the chamfer because
the pressure distribution of the surrounding ferromagnetic
fluid is strongly affected by the application of a
magnetic field. An augmented pressure force arises within
the fluid which is directed toward decreasing values of
field magnitude, i.e., the stronger the field, the greater
the pressure. In the present invention, since the magnet
is surrounded on all sides by the ferromagnetic fluid, it
tends absent the force of gravity) toward equilibrium at
the center ox the chamber. Any force which operates to
bring the magnet and any interior surface of the chamber
into closer proximity with each other will, as a
US consequence of such movement. also cause a concentration
of flux lines within the reduced volume ox ferromagnetic
fluid then remaining immediately between the chamfer
surface and the magnet. This localized compression of the
magnetic field is accompanied by a localized increase in
pressure which provides the force necessary to restore
equilibrium.
The chamber which contains the magnet and the
ferromagnetic fluid is adapted to provide at least one
pickup coil which it oriented in a plane substantially
perpendicular to the magnet's lines of flux. Since the
magnet has relatively high inertia as compared to the
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inertia of the chamber and coil assembly, and since the
ferromagnetic fluid advantageously presents a relatively
low mechanical impedance to relative movement between the
two components, the chamber and coil assembly, being in
contact with the surrounding medium, is free to respond to
the velocity of the particles of that surrounding medium
as they oscillate about their equilibrium position. These
oscillatory movements cause the coil to cut the magnet's
lines of flux thereby inducing in the coil an
electromotive force (or flux change) proportional to the
velocity of the coil relative to the magnet.
Being directionally sensitive, the geophones of this
invention produce a much greater flux change in the coil
lo for movement along the axis of sensitivity than for
movement in other directions. Iris is due to the
previously described asymmetrical relationship between the
magnetic field and the coil.
In one embodiment of this invention, asymmetry is
introduced by allowing a magnet to be displaced downwardly
by the force of gravity until a new elbowroom point
within the ferromagnetic fluid is reached. In other
embodiments of this invention, the magnet is weighted
eccentrically and gravity causes it to rotate about a
horizontal axis in a manner analogous to the use of
gimbals. In this latter case, the asymmetry is magnetic
in nature and results from the use of a composite magnet
wherein one half of the magnet is of opposite polarity
from the other half, rather than an asymmetry in the
magnet's physical potion relative to the coil.
It is thus seen thaw, as used in this invention, the
; ferromagnetic fluid has multifaceted utility. It
facilitates the asymmetry necessary for directionality, it
provides a gimbal-like action to permit self-orientation,
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and it acts as an omnidirectional spring with low mechanical
impedance to permit response to seismic signals.
The ferromagnetic fluid has one drawback, however.
After prolonged storage without movement (weeks or months) there
is a tendency for the magnetic particles to segregate under the
influence of the magnetic fluid gradient. This can cause the
magnet to become locked in place, and unable to stir the fluid to
restore the previous uniformity. The problem can be overcome by
the use of specially stabilized fluids, combined with a shaker or
vibrator through which the streamer is passed during deployment.
Although not free to move under the influence of small vibrations,
the magnet is not so firmly locked that vigorous shaking cannot
move it. The stabilized fluid responds to the shaking by reds-
pursing and restoring the normal floating action.
Accordingly, the present invention provides a geophone
comprising, in combination: magnetic field means with a polar
axis aligned horizontally; sealed chamber means containing a ferry-
magnetic fluid and said magnetic field means, the ferromagnetic
fluid interacting with the magnetic fluid means to movably suspend
said magnetic field means within the ferromagnetic fluid; coil
means associated with said sealed chamber means, the axis of the
coil means aligned with the axis of the magnetic field means where-
by relative vertical movements of the sealed chamber means and the
magnetic means electromagnetically induce a current within the
coil means; and connection means for electrically connecting said
coil to a conductor means.
The disclosed particle velocity sensing transducer or
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geophone is directionally sensitive and self-orienting, is of
small size, simple design and low cost to the user making it
economically feasible to incorporate great numbers of these go-
phones into the construction of marine seismic detection streamers.
A vertically sensitive self-orientiny geophone may be
advantageously combined with pressure-sensitive hydrophores to
obtain a unidirectional or corroded response.
The self-orienting geophone may, at the time of menu-
lecture, be configured to produce sensitivity in any desired
direction, I, vertical, horizontal or 45 degrees to either side
of vertical, thereby making possible the construction of marine
seismic detection streamers which can provide the information
necessary to laterally locate the source of the reflected signal
with respect to the streamer. The self-orienting geophone does
not require the use of mechanical bearing gimbals or slip ring
and brush electrical contacts, and does not require internal
maintenance or adjustment after installation in a marine seismic
survey streamer.
GRIEF DESCRIPTION OF DRAWINGS
Figure 1 is a perspective view which is partially broken
away to show a cross section of the functional components of one
embodiment of the invention.
Figure 2 is a longitudinal elevation Al view of the
chamber and coil assembly which is common to each illustrated
embodiment of the invention.
Figure 3 is a diagrammatic transverse cross section of
the embodiment shown in Figure 1 taken along cutting line A-A of
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Figure 2.
Figure 4 is a diagrammatic transverse cross section
taken along cutting line A-A of Figure 2 of an alternative
embodiment of the invention (with coils omitted for clarity) which
is configured to provide vertical sensitivity.
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Figure 5 is a diagrammatic transverse cross section
taken along cutting line A-A of Figure 2 of still another
alternative embodiment of the invention (also with the
coils omitted for clarity) which, in this instance, is
configured to provide horizontal sensitivity.
BYWAY RODE OF CARRYING OMIT TOE INVENTION
'rho basic structure of this invention, as shown in
Figure 1, comprises a magnet flatten inside a chamber
filled with magnetic fluid together with external wire
coils which interact with the flux of the magnet. the
three embodiments shown in the drawings differ in
construction primarily on the basis of the configuration
of the magnetic structure whence magnetic field) utilized.
As best seen in Figure 1, the chamber 12 is a sealed
compartment with relatively thin walls which may be in the
form of a right circular cylinder 16 with parallel ends
17, 17 disposed in planes which are perpendicular to the
axis of the chamber. Each end wall 17 supports a coil of
wire 14 coaxial with said end wall.
Coils 14, 14 are conveniently mounted on flanged
coil forms 20, 20 which are shown formed as an integral
part of and coaxial with end walls 17, 17. However, it is
understood that coil form 20 could be formed as a separate
part and subsequently attached to the end wall 17, or a
preformed coil could be directly attached to end wall 17
with adhesive, thereby dispensing with coil form 20
altogether. Leads 15, 15 extend from each coil 14 to
permit electrical connection to the signal wires (not
shown) which extend throughout the length of the detection
streamer.
When installed within the oil-filled tubular
detection streamer, the invention it positioned with its
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end walls 17, 17 perpendicular to and coaxial with the
towing axis of the streamer designated by the arrow, T,
shown in Figure 1. In this position, chamber 12 and coils
14, I are fully immersed within the oil which fills the
detection streamer and are free to move in response to the
particle velocity of the displacement waves present in the
surrounding medium. communication with the surrounding
medium is thus accomplished without the need for
perforated structures, diauhracJms, machined and threaded
fittings, o-rings or other complicated structural
components.
Within the chamber 12 is a magnet 10 which may be a
solid magnet of the short cylindrical shave shown in
Figure 1. Magnet 10 is magnetized axially along tow axis
T as shown by the North (N) and South (S) pole
designations adjacent to the magnet's pole surfaces which
lie parallel to ends 17, 17. This causes the lines of
flux associated with magnet 10 to intercept the plane of
each coil 14 on a substantially perpendicular basis.
Magnet 10 is suspended at an equilibrium point
within chamber 12 by the pressure distribution ox the
surrounding ferromagnetic fluid. The narrower the space
between the surface ox magnet 10 and any given inner
surface of chamber 12, the treater the augmented pressure
which will exist between the two surfaces. Thus, the
narrow craps Al 18 which exist between the pole faces of
magnet 10 and the inner surfaces ox chamber end walls 17,
17 are associated with strong augmented pressure forces
which keep the magnet firmly centered between the two end
walls 17, 17 and which provide a Starr resistance to
relative movement between the Mooney 10 and chamber 12 in
response to axial vibrations along tow axis T or to
twisting motions and undulations of the detection streamer
as it is toyed through the water. This same augmented
pressure force also serves to suspend magnet 10 in the
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vertical direction as best shown in Figure 3.
In Figure 3 is shown a diagrammatic cross section of
the embodiment shown in Figure 1 taken along cutting line
A-A of Figure 2. This Figure 3 looks along tow axis T and
shows the equilibrium position of the magnet 10 with
respect to the chamber periphery 16 and coil 14. All
other details are eliminated to provide clarity. As
shown, the magnet 10 is floating off-center within the
chamber 12. issue is due to the large diameter of the
chamber 12 relative to that of the magnet 10 so that the
pressure augmentation necessary to balance the downward
displacement caused by gravity occurs only when magnet 10
has moved quite close to the chamber periphery 16. Coil
14, being attached to the chamber end wall 17, remains
coaxial relative to chamber periphery 16. The magnet 10
and the coils 14, 14 thus become asymmetrically related
due to the force of gravity.
Due to this vertically asymmetric relationship, the
quantity of magnetic flux from magnet 10 magnetized
perpendicular to the plane of Figure 3) that passes
through coils 14, 14 varies substantially when the chamber
and its attached coils move vertically in response to
displacement waves in the surrounding medium. This
induces an electromotive force in coils 14l 14
proportional to the velocity of coils I 14 relative to
magnet 10; a voltage output that can be amplified and
recorded by the usual methods However, if the relative
movement between coils 14, 14 and magnet 10 is horizontal,
the flux change in the coils is relatively slight due to
the holi~.ontally symmetrical relationship between magnet 10
and coils 14, 14.
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Thus, the structure of Figure 1 and 3 has the
response of a vertically oriented geophone, due oriole to
the effect of the force of gravity which causes the magnet
and coils to be asymmetrically related for vertical
particle movement and still remain symmetrically related
for horizontal particle movement. Also, should the
structure be rotated about Tenneco axis T, the
ferromaglletic fluid allows gravity to cause the magnet I
to immediately seek and then retain the preexisting
vertically asymmetric relationship between it and the
coils 14, 14. This self-orienting aspect of the invention
permits a plurality of the devices to be installed within
a detection streamer without concern for the changes in
orientation undergone by the streamer as it is towed
through the water. Each geophone constructed in
accordance with this invention will separately and
independently maintain its own directional orientation.
Since the only moving part of the structure is the magnet
which moves through the ferromagnetic fluid, the coils I
19 can be in permanent fixed electrical connection with
the signal wires in the streamer. The need for mechanical
gimbals, slip rings and electrical contact brushes is
advantageously eliminated.
on alternative embodiment is shown in diagrammatic
cross section in Figure 4 with the coils 14, 14 and coil
forms 20, 20 omitted for clarity of illustration. Roy
most significant difference between this embodisnent and
the embodiment previously described is found in the type
of magnetic structure utilized. AS seen in Figure 4, the
magnetic structure of this embodiment i preferably formed
as a relatively thick walled hollow, right circular
cylinder which is magnetically divided along its diameter
into two axially magnetized segments or halves, with each
half being of opposite polarity to the other.
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he axial magnetization of the segments is along tow
axis T and the opposite polarities of the segments are
shown in Figure 4 with the North (N) pole designation on
the facing pole surface of magnet segment 22, and the
South (S) pole designation on the facing pole surface of
magnet segment 24. The two segments together form
composite magnet 21. If composite magnet 21 were to be
viewed from the opposite direction along tow axis T, the
observed polarities of segment 22 and segment 24 would be
the opposite of those shown in Figure 4. It is understood
that the polarities shown have been arbitrarily assigned
to each segment and that the prerequisite for proper
functioning of the invention is that the two segments be
of opposite axial polarity ¦
Chamber 12 is filled with ferromagnetic fluid and
composite magnet 21 floats within chamber 12 for the same
reasons as previously described for the first embodiment.
However, in this second embodiment, composite magnet 21
has an outer diameter which is only slightly smaller than
the inside diameter of chamber 12. This causes composite
magnet 21 to assume a position which is substantially
concentric (i.e., displaced downwardly only slightly) in
the vertical direction with regard to the chamber I even
when subjected to the full force of gravity as is the case
when the device is mounter in a detection streamer which
is being towed. The coils 14, 14 of this second
embodiment are coaxial with chamber 12 and are OX
substantially the same inner and outer diameter as
composite magnet 21.
The hollow center 28 of composite magnet 21 has a
biasing means mounted within it. In the preferred form of
this second embodiment, the biasing jeans is a
nonmagnetic weight 26. Weight I may be made of lead or
other heavy non-magrletic substance, or it could be a
hollow structure filled with yes ox other light substance.
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In the case of use of a gas, etc., gravity would cause
composite magnet 21 to rotate 180 degrees from the
position depicted in Figure 4. Any convenient means may
be utilized to secure weight 26 into position within
hollow center 28. In the embodiment shown, weight 26 is
held in position by filling hollow center 28 with a
polymerized epoxy resin.
The pOSitiOIl of weight 26 within hollow center 28
determines the axis of sensitivity of the geophone
represented by this second embodiment of the invention.
In Figure 4, weight I is shown positioned along the
periphery of hollow center 28 and is centered over a point
which is equidistant from (or rotated 90 degrees from) the
two boundaries on either side of composite magnet 21 where
magnet section 22 meets magnet section 24. When so
positioned, gravity acts upon weight 26 to cause composite
magnet 21 to rotate until it reaches equilibrium. At
equilibrium, a true vertical line drawn from top to bottom
MU through the center of composite magnet 21 will bisect
magnet segments 22 and 24 and will pass through the center
of gravity of weight 26.
Thus, when weight 26 is positioned to produce
vertical sensitivity, the unweighed magnet segmerlt 22
becomes the "top half" and the weighted magnet segment 24
becomes the "bottom half". end, the boundaries between
the two halves coincide with true horizontal It is thus
seen that the axis of sensitivity of this embodiment is
perpendicular to a straight line which lies parallel to
the two boundaries between magnet section 22 and magnet
section 24. As in the first embodiment, should chamber 12
and its associated coils 14, 14 be rotated about towing
axis T, gravity alone will act upon weight 26 to cause
composite magnet 21 to rotate so that sensitivity to true
` vertical is maintained.
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: In this second embodiment, relative movement between
the composite magnet 21 and coils 14, 14 in the vertical
direction produces a large net change of flux in coils 14,
14 because of the opposite polarity of magnet segments 22
and 24, i.e., the magnetic field produced by composite
magnet 21 is asymmetric with respect to coils 14, 14 for
vertical movement. However, a relative horizontal
movement causes virtually no net change in flux in coils
14, 14, the cancellation being superior to that produced
by the first embodiment shown in Figure 1 and Figure 3 due
to the horizontal double symmetry of the structure of this
second embodiment.
A third alternative embodiment of this invention is
shown in diagrammatic cross section in Figure 5. Once
again, coils lo, 14 and coil forms 20, 20 are omitted for
clarity of illustration. The only difference between the
embodiment Shirley in Figure 5 from the one shown in figure
4 is the location of the biasing means, weight 26. Though
it still remains located on the periphery of hollow center
28, weight 26 is now centered over one of the boundaries
between magnet segment 22 and magnet segment 24. This
location is 90 degrees away from that utilized in the
second embodiment.
With weight 26 in this new location, composite
magnet 21 now reaches equilibrium in an orientation which
is rotated 90 degrees prom that shown for the second
embodiment The boundaries between magnet segment 22 and
magnet segment 24 now are parallel to true vertical rather
than true horizontal. Since the axis of sensitivity is
perpendicular to a straight line drawn through the two
boundaries, the embodiment shown in Figure 5 is primarily
sensitive to horizontal motion, and has little sensitivity
to vertical motion.
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It is thus seen from Figures 4 and 5 that the design
of the geophone of this invention may be modified to
display enhanced sensitivity along any desired axis by
merely changing the location of weight 26 along the
S periphery of hollow center 28. ire instance, placement of
weight 26 at a point one-half way between the two
locations previously shown results in a self-orientin~
geophone with a sensitivity for motion along a path which
is 45 degrees to one side of vertical. By reversing the
geophone end for end within the detection streamer, this
same production element will display a sensitivity for
motion along a path which is 95 degrees to the other side
of vertical.
lo oven if it were not possible to construct a device
with 45 degree sensitivity, the availability of a
horizontally sensitive self-orienting geophone, as shown
in Figure 5, broadens considerably the number of acoustic
responses possible from an array ox a plurality of the
devices. For example, an array containincJ equal numbers
ox vertically and horizontally sensitive geophone elements
constructed in accordance with this invention can have its
axis of sensitivity steered anywhere in the plane
perpendicular to tow axis T by electronically summincl
various proportions of the two signals corresponding to
horizontal and vertical motions. Sensitivity of the array
to sound traveling along the tow axis T is low, not only
because of the plurality of the elements, but also because
the augmented pressure within the ferromagnetic fluid
associated with axial gaps 18, 18 makes each individual
element insensitive to such longitudinal vibrations
Louvre, since it utilizes only geophones, this array is
equally sensitive to motion in both directions along the
axis of sensitivity of the two types of elements with a
resultant ambiguity as to the absolute direction of any
sound waves received.
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This directional ambiguity can be resolved by
including a plurality of pressure transducers or
hydrophores in the horizontal and vertical geophone array
described above. Three different signals would then be
available for electronic processing, either to generate a
corroded response in any chosen direction in the plane
perpendicular to tow axis T, or to analyze the absolute
direction of arrival of any packet of sound waves. Some
loss of the rejection of noise along tow axis T must occur
due to the inclusion of the nondirectional hydrophores,
but the pressure signal could be used to establish the
direction of propagation of a sound packet by using the
strongest waves in that packet, while the quieter
displacement wave signal from the self-orienting geophones
could be used to analyze the geologic substrata which
reflected the signal.
By providing a practical, self-orienting particle
velocity transducer which may be configured to produce
sensitivity along any desired axis within a vertical
plane, the yeo~hone of this invention makes possible the
construction of improved marine seismic detection streamer
arrays which are capable of producing a unidirectional
corroded response together with the further capability of
directing that corroded response to either side of the
streamer to provide information about the lateral position
of the sources of reflected sound with respect to the tow
axis of the streamer.
These same advantages exist if the discrete
horizontally and vertically sensitive yeophone elements
are replaced with geophones which incorporate 45 degree
sensitivity. In this latter case, only one production
version of the invention is required since the elements
need only be turned end for end to change the directional
sensitivity from one side of vertical to the other.
Seismic surveys made with either version of the above
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described streamers could thus be completed with a greatly
reduced number of truculency and hence reduced costs.
INDUSTRIAL APPLICABILITY
The geophones of this invention may be manufactured
by using existing machinery, processes and techniques.
The ferromagnetic fluid, epoxy resin, coil wire and each
type of magnet (preferably Samarium-cobalt) are all
existing products which are readily available in the
marketplace The shape of the lead weight which biases
the composite magnet is not critical, except that it
should lend itself to ease of assembly, and thus could be
crescent shaped in cross section with an indentation to
mark its center point instead of the round weight shown in
the drawings. Finally, the chamber and coil forms may be
readily wormed from a suitable plastic by the injection
molding process.
Major areas of use include seismic surveying and
particularly those systems used to evaluate underwater
geological formations. In a typical application, a
plurality of these geophones are installed in a
longitudinally spaced apart manner within an oil filled
flexible tube or streamer which can be in excess of 5/000
feet in length. Such an assembly is known as a detection
array The streamer also contains an electrical cable.
Subgroups of the geophones are connected to various wires
within the cable with each subgroup serving as a single
receiving channel. Being self-orienting, the geophones of
this invention may be paired with pressure sensitive
hydrophores to provide a unidirectional or corroded
response.
When not deployed for use, the streamer may be
conveniently stored on a large reel carries on board the
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survey vessel. Upon deployment, the array is towed along
a track line; a seismic shock is initiated and the
reflections of that shock emanating from the underlying
geological substrata are sensed by the geophones within
the array. eye various outputs from each channel are
electronically processed on board the towing vessel and
then utilized as inputs to a seismic recorder which
generates a seismic profile or chart.
In compliance with the statute, tile invention has
been described in language more or less specific as to
structural features. It is to be understood, however,
that the invention is not limited to the specific features
shown, since the means and construction herein disclosed
comprise a preferred form of putting the invention into
effect. The invention is, therefore, claimed in any of
its forms or modifications within the legitimate and valid
scope of the appended claims appropriately interpreted in
accordance with the doctrine of equivalents.
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